Time-Domain Electromagnetic Methods

Basic Concept

Conventional DC resistivity techniques have been applied for many
years to a variety of geotechnical applications. More
recently, electromagnetic techniques, with different advantages
(and disadvantages) compared with conventional DC, have been used
effectively to measure the resistivity (or its reciprocal, the
conductivity) of the Earth.

Electromagnetic techniques can be broadly divided into two
groups. In frequency-domain instrumentation (FDEM), the
transmitter current varies sinusoidally with time at a fixed
frequency that is selected on the basis of the desired depth of
exploration of the measurement (high frequencies result in
shallower depths). In most time-domain (TDEM)
instrumentation, on the other hand, the transmitter current,
although still periodic, is a modified symmetrical square wave,
as shown in figure 1. It is seen that after every second
quarter-period, the transmitter current is abruptly reduced to
zero for one quarter period, whereupon it flows in the opposite
direction.

Figure 1. Transmitter current wave form.

A typical TDEM resistivity sounding survey configuration is shown
in figure 2, where it is seen that the transmitter is connected
to a square (usually single turn) loop of wire laid on the
ground. The side length of the loop is approximately equal
to the desired depth of exploration, except that for shallow
depths (less than 40 m), the length can be as small as 5 to
10 m in relatively resistive ground. A multi-turn receiver
coil, located at the center of the transmitter loop, is connected
to the receiver through a short length of cable.

Figure 2. Central loop sounding configuration.

The principles of TDEM resistivity sounding are relatively easily
understood. The process of abruptly reducing the
transmitter current to zero induces, in accord with Faraday's
law, a short-duration voltage pulse in the ground, which causes a
loop of current to flow in the immediate vicinity of the
transmitter wire, as shown in figure 3. In fact,
immediately after transmitter current is turned off, the current
loop can be thought of as an image in the ground of the
transmitter loop. However, because of finite ground
resistivity, the amplitude of the current starts to decay
immediately. This decaying current similarly induces a
voltage pulse that causes more current to flow, but now at a
larger distance from the transmitter loop, and also at greater
depth, as shown in figure 3. This deeper current flow also
decays due to finite resistivity of the ground, inducing even
deeper current flow and so on. The amplitude of the current
flow as a function of time is measured by measuring its decaying
magnetic field using a small multi-turn receiver coil usually
located at the center of the transmitter loop. From the
above, it is evident that, by making measurement of the voltage
out of the receiver coil at successively later times, measurement
is made of the current flow and thus also of the electrical
resistivity of the earth at successively greater depths.
This process forms the basis of central loop resistivity sounding
in the time domain.

Figure 3. Transient current flow in the ground.

The output voltage of the receiver coil is shown schematically
(along with the transmitter current) in figure 4. To
accurately measure the decay characteristics of this voltage, the
receiver contains 20 narrow time gates (indicated in figure 5),
each opening sequentially to measure (and record) the amplitude
of the decaying voltage at 20 successive times. Note that
to minimize distortion in measurement of the transient voltage,
the early time gates, which are located where the transient
voltage is changing rapidly with time, are very narrow, whereas
the later gates, situated where the transient is varying more
slowly, are much broader. This technique is desirable since
wider gates enhance the signal-to-noise ratio, which becomes
smaller as the amplitude of the transient decays at later
times. It will be noted from figure 4 that there are four
receiver voltage transients generated during each complete period
(one positive pulse plus one negative pulse) of transmitter
current flow. However, measurement is made only of those
two transients that occur when the transmitter current has just
been shut off, since in this case, accuracy of the measurement is
not affected by small errors in location of the receiver
coil. This feature offers a very significant advantage over
FDEM measurements, which are generally very sensitive to
variations in the transmitter coil/receiver coil spacing since
the FDEM receiver measures while the transmitter current is
flowing. Finally, particularly for shallower sounding,
where it is not necessary to measure the transient
characteristics out to very late times, the period is typically
of the order of 1 msec or less, which means that in a total
measurement time of a few seconds, measurement can be made and
stacked on several thousand transient responses. This is
important since the transient response from one pulse is
exceedingly small, and it is necessary to improve the
signal-to-noise ratio by adding the responses from a large number
of pulses.

Figure 4. Receiver output wave form.

Apparent Resistivity In TDEM Soundings

Figure 5 shows, schematically, a linear
plot of typical transient response from the earth. It is
useful to examine this response when plotted logarithmically
against the logarithm of time, particularly if the earth is
homogeneous (i.e. the resistivity does not vary with either
lateral distance or depth). Such a plot is shown in figure
303, which suggests that the response can be divided into an
early stage (where the response is constant with time), an
intermediate stage (response shape continually varying with
time), and a late stage (response is now a straight line on the
log-log plot). The response is generally a mathematically
complex function of conductivity and time; however, during the
late stage, the mathematics simplifies considerably, and it can
be shown that during this time the response varies quite simply
with time and conductivity
as

(1)

where

e(t)= output
voltage from a single-turn receiver coilof area 1 m2

k1 = a constant

M = product of Tx current x area
(a-m2)

σ = terrain conductivity (siemens/m = S/m =
1/Ωm)

t = time (s)

Unlike the case for conventional resistivity
measurement, where the measured voltage varies linearly with
terrain resistivity, for TDEM, the measured voltage e(t) varies
as σ3/2, so it is intrinsically more sensitive
to small variations in the conductivity than conventional
resistivity. Note that during the late stage, the measured
voltage is decaying at the rate t-5/2, which is very
rapidly with time. Eventually the signal disappears into
the system noise, and further measurement is impossible.
This is the maximum depth of exploration for the particular
system.

In order to obtain the resistivity of the ground, equation 75a is
rearranged (inverted) to give equation 75b:

(2b)

If ground resistivity is uniform as the
interelectrode spacing (a) is
increased, the measured voltage increases directly withaso that the right-hand side of equation 2b stays constant,
and the equation gives the true resistivity. Suppose now
that the ground is horizontally layered (i.e., that the
resistivity varies with depth); for example, it might consist of
an upper layer of thicknesshand
resistivity ρ1, overlying a more resistive basement of
resistivity ρ2 > ρ1, (this is called a two-layered
earth). At very short interelectrode spacing
(a<<h), virtually
no current penetrates into the more resistive basement, and
resistivity calculation from equation 2b will give the value
ρ1. As interelectrode spacing is increased, the current
(I) is forced to flow to greater and greater depths.
Suppose that, at large values ofa(a>>h), the effect
of the near-surface material of resistivity ρ1 will be
negligible, and resistivity calculated from equation 2b will give
the value ρ2, which is indeed what happens. At
intermediate values ofa(a.
h), the resistivity given by equation
2b will lie somewhere between ρ1 and ρ2.

Equation 2b is, in the general case, used to
define an apparent resistivity ρa(a), which is a function of
a. The variation ofaρa(a) witha

(3)

is descriptive of the variation of resistivity with depth.
The behavior of the apparent resistivity ρa(a) for
a Wenner array for the two-layered earth above is shown
schematically in figure 7. It is clear that in conventional
resistivity sounding, to increase the depth of exploration, the
interelectrode spacing must be increased. In the case of TDEM
soundings, on the other hand, it was observed earlier that as
time increased, the depth to the current loops increased, and
this phenomenon is used to perform the sounding of resistivity
with depth. Thus, in analogy with equation 3, equation 4 is
inverted to read (since ρ = 1/σ)

(4)

Suppose once again that terrain resistivity does
not vary with depth (i.e. a uniform half-space) and is of
resistivity ρ1. For this case, a plot of
ρa(t) against
time would be as shown in figure 8. Note that at late time
the apparent resistivity ρa(t) is equal to
ρ1, but at early time ρa(t) is much
larger than ρ1. The reason for this is that the
definition of apparent resistivity is based (as seen from figure
6) on the time behavior of the receiver coil output voltage at
late time when it decays ast-5/2. At earlier and intermediate time,
figure 6 shows that the receiver voltage is too low (the dashed
line indicates the voltage given by the late stage approximation)
and thus, from equation 4, the apparent resistivity will be too
high. For this reason, there will always be, as shown on
figure 8, a "descending branch" at early time where the apparent
resistivity is higher than the half-space resistivity (or, as
will be seen later, is higher than the upper layer resistivity in
a horizontally layered earth). This is not a problem, but
it is an artifact of which we must be aware.

If we let the earth be two-layered of upper layer
resistivity ρ1, and thicknessh, and
basement resistivity ρ2 (>ρ1), at early time when the
currents are entirely in the upper layer of resistivity ρ1
the decay curve will look like that of figure 6, and the apparent
resistivity curve will look like figure 7. However, later
on the currents will lie in both layers, and at much later time,
they will be located entirely in the basement, of resistivity
ρ2. Since ρ2>ρ1, equation 4 shows that, as
indicated in figure 9a, the measured voltage will now be less
than it should have been for the homogeneous half-space of
resistivity ρ1. The effect on the apparent resistivity
curve is shown in figure 10a. Since at late times all the
currents are in the basement, the apparent resistivity
ρa(t) becomes
equal to ρ2, completely in analogy for figure 7 for
conventional resistivity measurements. In the event that
ρ2<ρ1, the inverse behavior is also as expected, i.e.,
at late times the measured voltage response, shown in figure 9b,
is greater than that from a homogeneous half-space of resistivity
ρ1, and the apparent resistivity curve correspondingly
becomes that of figure 10b, becoming equal to the new value of
ρ2 at late time. Note that for the case of a
(relatively) conductive basement, there is a region of
intermediate time (shown ast*), where the
voltage response temporarily falls before continuing on to adopt
the value appropriate to ρ2. This behavior, which is a
acteristic of TDEM, is again not a problem, as long as it is
recognized. The resultant influence of the anomalous
behavior on the apparent resistivity is also shown on figure 10b
att*.

To summarize, except for the early-time descending branch and the
intermediate-time anomalous region described above, the sounding
behavior of TDEM is analogous to that of conventional DC
resistivity if the passage of time is allowed to achieve the
increasing depth of exploration rather than increasing
interelectrode spacing.

Curves of apparent resistivity such as figure 10 tend to disguise
the fact, that, at very late times, there is simply no signal, as
is evident from figure 9. In fact, in the TDEM central loop
sounding method, it is unusual to see, in practical data, the
curve of apparent resistivity actually asymptote to the basement
resistivity due to loss of measurable signal. Fortunately,
both theoretically and in practice, the information about the
behavior of the apparent resistivity curve at early time and in
the transition region is generally sufficient to allow the
interpretation to determine relatively accurately the resistivity
of the basement without use of the full resistivity-sounding
curve.

Data Acquisition

A common survey configuration consists of a
square single turn loop with a horizontal receiver coil located
at the center. Data from a resistivity sounding consist of
a series of values of receiver output voltage at each of a
succession of gate times. These gates are located in time
typically from a few microseconds up to tens or even hundreds of
milliseconds after the transmitter current has been turned off,
depending on the desired depth of exploration. The receiver
coil measures the time rate of change of the magnetic field
e(t)2=dB/dt, as a function of time during the
transient. Properly calibrated, the units of e(t) are
V/m2 of receiver coil area; however, since the
measured signals are extremely small, it is common to use
nV/m2, and measured decays typically range from many
thousands of nV/m2 at early times to less than 0.1
nV/m2 at late times. Modern receivers are
calibrated in nV/m2 or V/m2. To check
the calibration, a "Q-coil," which is a small short-circuited
multi-turn coil laid on the ground at an accurate distance from
the receiver coil, is often used so as to provide a transient
signal of known amplitude.

The two main questions in carrying out a resistivity sounding are
(a) how large should the side lengths of the (usually
single-turn) transmitter be, and (b) how much current should the
loop carry? Both questions are easily answered by using one
of the commercially available forward layered-earth computer
modeling programs. A reasonable guess as to the possible
geoelectric section (i.e., the number of layers, and the
resistivity and thickness of each) is made. These data are
then fed into the program, along with the proposed loop size and
current, and the transient voltage is calculated as a function of
time. For example, assume that it is suspected that a clay
aquitard may exist at a depth of 20 m in an otherwise clay-free
sand. Resistivity of the sand might be 100 Ωm, and
that of the clay layer 15 Ωm. Desired information
includes the minimum thickness of the clay layer that is
detectable, and the accuracy with which this thickness can be
measured. The depth of exploration is of the order of the
loop edge size, so 10 m by 10 m represents a reasonable guess for
model calculation, along with a loop current of 3 A, which is
acteristic of a low-power, shallow-depth transmitter.
Before doing the calculations, one feature regarding the use of
small (i.e., less than 60 m by 60 m) transmitter loops for
shallow sounding should be noted. In these small loops, the
inducing primary magnetic field at the center of the loop is very
high, and the presence of any metal, such as the receiver box, or
indeed the shielding on the receiver coil itself, can cause
sufficient transient response to seriously distort the measured
signal from the ground. This effect is greatly reduced by
placing the receiver coil (and receiver) a distance of about
10 m outside the nearest transmitter edge. As shown
later, the consequence of this on the data is relatively small.

The first task is to attempt to resolve the difference between,
for example, a clay layer 0 m thick (no clay) and 1 m
thick. Results of the forward layered-earth calculation,
shown in figure 11, indicate that the apparent resistivity curves
from these two cases are well separated (difference in calculated
apparent resistivity is about 10%) over the time range from about
8 μs to 100 μs, as would be expected from the
relatively shallow depths. Note that to use this early time
information, a receiver is required that has many narrow early
time gates in order to resolve the curve, and also has a wide
bandwidth so as not to distort the early portions of the
transient decay. Note from the figure that resolving
thicknesses from 1 to 4 m and greater will present no problem.

Figure 12. Forward layered-earth calculations

Having ascertained that the physics of TDEM sounding will allow
detection of this thin layer, the next test is to make sure that
the 10‑ by 10‑m transmitter running at 3 A will
provide sufficient signal-to-noise over the time range of
interest (8 to 100 μs). The same forward layered-earth
calculation also outputs the actual measured voltages that would
be measured from the receiver coil. These are listed (for
the case of thickness of 0 m, which will produce the lowest
voltage at late times) in Seismic
Methods Table 1. Focus attention on the first
column (which gives the time, in seconds) and the third column
(which gives the receiver output as a function of time, in
V/m2). The typical system noise level
(almost invariably caused by external noise sources) for gates
around 100 to 1,000 μs is about 0.5 nV/m2,
or 5 x 10-10V/m2. From columns
1 and 3 see that, for the model chosen, the signal falls to 5 x
10-10V/m2 at a time of about 630
μs and is much greater than this for the early times when the
apparent resistivity curves are well-resolved, so it is learned
that the 10‑ by 10‑m transmitter at 3 A is entirely
adequate. In fact, if a 5‑ by 5‑m transmitter
was used, the dipole moment (product of transmitter current and
area) would fall by 4, as would the measured signals, and the
signal-to-noise ratio would still be excellent over the time
range of interest. Thus assured, assuming that the model
realistically represents the actual conditions of resistivity,
the procedure will be able to detect the thin clay layer.
Before proceeding with the actual measurement, it would be wise
to vary some of the model parameters, such as the matrixand clay
resistivities, to see under what other conditions the clay will
be detectable. The importance of carrying out such
calculations cannot be overstated. The theory of TDEM
resistivity sounding is well understood, and the value of such
modeling, which is inexpensive and fast, is very high.

It was stated above that offsetting the receiver coil from the
center of the transmitter loop would not greatly affect the shape
of the apparent resistivity curves. The reason for this is
that the vertical magnetic field arising from a large loop of
current (such as that shown in the ground at late time in figure
3) changes very slowly in moving around the loop center.
Thus, at late time, when the current loop radius is significantly
larger than the transmitter loop radius, it would be expected
that moving the receiver from the center of the transmitter loop
to outside the loop would not produce a large difference, whereas
at earlier times, when the current loop radius is approximately
the same as the transmitter radius, such offset will have a
larger effect. This behavior is illustrated in figure 13,
which shows the apparent resistivity curves for the receiver at
the center and offset by 15 m from the center of the
10‑ by 10‑m transmitter loop. At late time, the
curves are virtually identical.

One of the big advantages of TDEM geoelectric sounding over
conventional DC sounding is that for TDEM, the overall width of
the measuring array is usually much less than the depth of
exploration, whereas for conventional DC sounding, the array
dimension is typically (Wenner array) of the order of three times
the exploration depth. Thus, in the usual event that the
terrain resistivity is varying laterally, TDEM sounding will
generally indicate those variations much more accurately.
If the variations are very closely spaced, one might even take
measurements at a station spacing of every transmitter loop
length. It should be noted that most of the time spent
doing a sounding (especially deeper ones where the transmitter
loop is large) is utilized in laying out the transmitter
loop. In this case, it can be much more efficient to have
one or even two groups laying out loops in advance of the survey
party, who then follow along with the actual transmitter,
receiver, and receiver coil to make the sounding in a matter of
minutes, again very favorable compared with DC sounding. A
further advantage of TDEM geoelectric sounding is that, if a
geoelectric interface is not horizontal, but is dipping, the TDEM
still gives a reasonably accurate average depth to the
interface. Similarly, TDEM sounding is much less sensitive
(especially at later times) to varying surface topography.

It was explained above that, particularly at later times, the
shape of the apparent resistivity curve is relatively insensitive
to the location of the receiver coil. This feature is
rather useful when the ground might be sufficiently inhomogeneous
to invalidate a sounding (in the worst case, for example, due to
a buried metallic pipe). In this case, a useful and quick
procedure is to take several measurements at different receiver
locations, as shown in figure 14. Curve 5 is obviously
anomalous, and must be rejected. Curves 1-4 can all be used
in the inversion process, which handles both central and offset
receiver coils. Another useful way to ensure, especially
for deep soundings, that the measurement is free from errors
caused by lateral inhomogeneities (perhaps a nearby fault
structure) is to use a three- component receiver coil, which
measures, in addition to the usual vertical component of the
decaying magnetic field, both horizontal components. When
the ground is uniform or horizontally layered, the two horizontal
components are both essentially equal to zero, as long as the
measurement is made near the transmitter loop center (which is
why the technique is particularly relative to deep
sounding). Departures from zero are a sure indication of
lateral inhomogeneities that might invalidate the sounding.

Most receivers, particularly those designed for shallower
sounding, have an adjustable base frequency to permit changing
the length of the measurement time. With reference to
figures 1 and 4, changing the base frequency fb will
change the period T (T=1/fb), and thus the measurement
duration T/4. For transients that decay quickly, such as
shallow sounding, the measurement period, which should be
of the order of duration of the transient, should be short, and
thus the base frequency high. This has the advantage that,
for a given total integration time of, say 5 s, more transient
responses will be stacked, to improve the signal-to-noise ratio
and allow the use of smaller, more mobile, transmitter loops,
increasing survey speed. On the other hand, for deep
sounding, where the response must be measured out to very long
time, it is clear that the measurement period must be greatly
extended so that the transient response does not run on to the
next primary field cycle or indeed the next transient response,
and thus the base frequency must be significantly reduced.
The signal-to-noise ratio will deteriorate due to fewer
transients being stacked, and must be increased by either using a
larger transmitter loop and transmitter current (to increase the
transmitter dipole) and/or integrating the data for a longer
stacking time, perhaps for 30 s or even a minute. It should
be noted that should such run-on occur because too high a base
frequency was employed, it can still be corrected for in modern
data inversion programs; however, in extreme cases, accuracy and
resolution of the inversion will start to deteriorate.

In figure 4 and in this discussion, it was assumed that the
transmitter current is turned off instantaneously. To
actually accomplish this with a large loop of transmitter wire is
impossible, and modern transmitters shut the current down using a
very fast linear ramp. The duration of this ramp is
maintained as short as possible (it can be shown to have an
effect similar to that of broadening the measurement gate widths)
particularly for shallow sounding where the transient decays very
rapidly at early times. The duration of the transmitter
turn-off ramp (which can also be included in modern inversion
programs) is usually controlled by transmitter loop size and/or
loop current.

Sources of Noise

Noise sources for TDEM soundings can be divided into four
categories:

a) Circuit
noise (usually so low in modern receivers as to rarely cause a
problem).

b)
Radiated and induced noise.

c)
Presence of nearby metallic structures.

d)
Soil electrochemical effects (induced polarization).

Radiated noise consists of signals generated by radio and radar
transmitters and also from thunderstorm lightning activity.
The first two are not usually a problem; however, on summer days
when there is extensive local thunderstorm activity, the
electrical noise from lightning strikes (similar to the noise
heard on AM car radios) can cause problems, and it may be
necessary to increase the integration (stacking) time or, in
severe cases, to discontinue the survey until the storms have
passed by or abated.

The most important source of induced noise consists of the
intense magnetic fields from 50- to 60-Hz power lines. The
large signals induced in the receiver from these fields (which
fall off more or less linearly with distance from the power line)
can overload the receiver if the receiver gain is set to be too
high, and thus cause serious errors. The remedy is to
reduce the receiver gain so that overload does not occur,
although in some cases, this may result in less accurate
measurement of the transient since the available dynamic range of
the receiver is not being fully utilized. Another
alternative is to move the measurement array further from the
power line.

The response from metallic structures can be very
large compared with the response from the ground.
Interestingly, the power lines referred to above can often also
be detected as metallic structures, as well as sources of induced
noise. In this case, they exhibit an oscillating response
(the response from all other targets, including the earth, decays
monotonically to zero). Since the frequency of oscillation
is unrelated to the receiver base frequency, the effect of power
line structural response is to render the transient "noisy" as
shown in figure 15. Since these oscillations arise from
response to eddy currents actually induced in the power line by
the TDEM transmitter, repeating the measurement will produce an
identical response, which is one way that these oscillators are
identified. Another way is to take a measurement with the
transmitter turned off. If the "noise" disappears, it is a
good indication that power-line response is the problem.
The only remedy is to move the transmitter farther from the power
line. Other metallic responses, such as those from buried
metallic trash or pipes, can also present a problem, a solution
for which was discussed in the previous section (multiple
receiver sites, as shown in figure 14). If the response is
very large, another sounding site must be selected.
Application of another instrument such as a metal detector or
ground conductivity meter to quickly survey the site for pipes
can often prove useful.

A rather rare effect, but one which can occur, particularly in
clayey soils, is that of induced polarization. Rapid
termination of the transmitter current can ge up the minute
electrical capacitors in the soil interfaces (induced
polarization). These capacitors subsequently disge,
producing current flow similar to that shown in figure 3, but in
the opposite direction. The net effect is to reduce the
amplitude of the transient response (thus increasing the apparent
resistivity) or even, where the effect is very severe, to cause
the transient response to become negative over some range of the
measurement time. Since these sources of reverse current
are localized near the transmitter loop, using the offset
configuration usually reduces the errors caused by them to small
values.

In summary, it should be noted that in TDEM
soundings, the signal-to-noise ratio is usually very good over
most of the time range. However, in general, the transient
response is decaying extremely rapidly (of the order oft-5/2, or by a factor of about
300 for a factor of 10 increase in time). The result is
that toward the end of the transient, the signal-to-noise ratio
suddenly deteriorates completely, and the data become exceedingly
noisy. The transient is over!

Figure 15. Oscillations induced in receiver response by
power line.

Data Processing and Interpretation

In the early days of TDEM sounding, particularly in Russia where
the technique was developed (Kaufman and Keller 1983), extensive
use was made of numerically calculated apparent resistivity
curves for a variety of layered earth geometrics. Field
data would be compared with a selection of curves, from which the
actual geoelectric section would be determined. More
recently, the advent of relatively fast computer inversion
programs allows field transient data to be automatically inverted
to a layered earth geometry in a matter of minutes. An
inversion program offers an additional significant advantage.
All electrical sounding techniques (conventional DC,
magneto-telluric, TDEM) suffer to a greater or less extent from
equivalence, which basically states that, to within a given
signal-to-noise ratio in the measured data, more than one
specific geoelectric model will fit the measured data. This
problem, which is seldom addressed in conventional DC soundings,
is one of which the interpreter must be aware, and the advantage
of the inversion program is that, given an estimate of the
signal-to-noise ratio in the measured data, the program could
calculate a selection of equivalent geoelectric sections that
will also fit the measured data, immediately allowing the
interpreter to decide exactly how unique his solution really
is. Equivalence is a fact of life, and must be included in
any interpretation.

Advantages and Limitations

The advantages of TDEM geoelectric sounding over conventional DC
resistivity sounding are significant. They include the
following:

1. Improved speed of
operation.

2. Improved lateral
resolution.

3. Improved resolution of
conductive electrical equivalence.

4. No problems injecting
current into a resistive surface layer.

The limitations of TDEM techniques are as follows:

1. Do not work well in very
resistive material.

2. Interpretational material
for TDEM on, for example, 3D structures is still under
development.

3. TDEM equipment tends to be
somewhat more costly due to its greater complexity.

As mentioned above, the advantages are significant, and TDEM is
becoming a widely used tool for geoelectrical sounding.

Two Dimensional Imaging

As in multiple other geophysical techniques, and with the advent of computers with greater processing strengths, two-dimensional imaging using the one dimensional approaches, has become common place. By completing a series of one dimensional soundings, a two dimensional image can be generated from the resulting data. Figure 16 presents an example of a two dimensional image generated from a series of one dimensional, inverted TEM soundings, and resulting interpretation.

Figure 16. Two dimensional imaging results using stitched together one dimensional inversion results for a 5 layer model These images are presented for references purposes only, no endorsement of the software is intended.

The pages found under Surface Methods and Borehole Methods
are substantially based on a report produced by the United States Department of Transportation: